The invention relates to spacecraft, in particular satellites. More particularly, the invention relates to storing energy and controlling attitude by means of momentum wheels in satellites.
Satellites are commonly provided with solar panels which serve to provide electrical power for the equipment on board the satellite. Because the sun is eclipsed, energy storage devices are also provided so as to continue providing electricity when the satellite is in a position from which the sun is not visible. Proposals have been made to use batteries for this purpose, or energy storage devices that rely on momentum wheels. Momentum wheels store energy in the form of kinetic energy of rotation. The advantage of devices using momentum wheels, apart from their specific energy storage characteristics, is that they can also be used to alter the attitude of the satellite by acting on the speeds of rotation of the wheels variations in the speeds of rotation of the wheels give rise to variations in angular momentum that generate torque and thus enable the attitude of the satellite to be varied.
EP-A-0 922 636 proposes such an energy storage device based on momentum wheels; the introduction of that document sets out the advantages of momentum wheels compared with batteries, and the general principle on which momentum wheels operate. In that document, a device is proposed that uses pairs of wheels rotating in opposite directions; thus, when both wheels in a pair are in alignment and rotating at the same speed, the net angular momentum is zero. The mechanism has three pairs of wheels aligned on three axes in an orthogonal frame of reference, plus a fourth pair of wheels positioned so as to be capable of replacing any one of the other pairs in the event of failure. That document therefore proposes a mechanism having eight momentum wheels.
EP-A-0 849 170 describes another device for storing energy by momentum wheels. In that document, it is proposed to use one pair of momentum wheels with aligned axes, or three pairs of wheels on three orthogonal directions, or indeed a pyramid configuration.
U.S. Pat. No. 5,611,505 describes another energy storage device using momentum wheels. In that document, proposals are made to use a pair of momentum wheels; each of the wheels is mounted on a gimbal suspension with degrees of freedom about two axes perpendicular to the axis of rotation of the wheel.
In a presentation to NTIS, Flywheel Energy Storage Workshop, Oak Ridge, December 1995, TRW describes various configurations of momentum wheels, enabling four degrees of freedom to be obtained in an energy storage system using momentum wheels; the four degrees of freedom correspond to three components of the momentum, which is a vector, and to the energy stored which is a scalar. A first configuration uses two wheels mounted on gimbals, the axis of rotation of each wheel being movable with a single degree of freedom. A second configuration uses two wheels in alignment and mounted head to head, on a suspension allowing the axis of rotation of the wheels to move with two degrees of freedom. A third configuration uses four wheels disposed on four lines converging on the apex of a pyramid.
In a presentation to the 17th Annual Space Power Workshop, Long Beach, Calif., Apr. 19-22, 1999, The Charles Stark Draper Laboratory Inc. proposed various configurations of momentum wheels. For a pair of contrarotating wheels, a first configuration is to provide a suspension having two degrees of freedom for each wheel, with suspension axes perpendicular to the axis of rotation of each wheel. Still for two wheels, a second configuration consists in providing a suspension having one degree of freedom about an axis perpendicular to the axis of rotation for each of the wheels, and a suspension having one degree of freedom about the axis of rotation for the pair of wheels. A third configuration uses three pairs of contrarotating wheels on three fixed orthogonal axes. In a fourth configuration, four wheels are disposed with axes of rotation that are perpendicular to the faces of a tetrahedron. A fifth configuration uses three wheels mounted on suspensions having one degree of freedom. The three axes of rotation intersect and are coplanar at rest, and the suspensions enable two of the wheels to move their axes in the coplanar plane and the third wheel to move perpendicularly to the plane.
The problem of the invention is to propose an energy storage system using momentum wheels which presents a minimum number of wheels, which provides four degrees of freedom, and which can accommodate failure of at least one wheel while continuing to operate in satisfactory manner.
In addition, the invention solves the novel problem of device symmetry regardless of which wheel should fail. Prior art systems are often symmetrical; however, symmetry is lost in the event of a wheel failing, and after failure some configurations are unfavorable for storing energy; the energy stored in a wheel can require a speed of about 100,000 revolutions per minute (rpm), and that generates significant angular momentum; this momentum is compensated only by rotation in the opposite direction of another wheel. The failure of one wheel in a pair limits the speed at which the remaining wheel of the pair can rotate, and thus the amount of energy that can be stored.
Unlike that, in the invention, in the event of a failure, proper operation continues to be ensured; in particular, the quantity of energy that can be stored does not decrease in the event of a failure.
The invention thus makes it possible to store energy in a minimum volume with a minimum mass for a given quantity of energy. The energy also makes it possible to conserve satisfactory operation even in the event of the failure of one wheel, and possibly even of two wheels.
More precisely, the invention proposes a momentum wheel energy storage device having at least five wheels whose axes of rotation are substantially parallel to the edges of a tetrahedron.
Advantageously, the device has six wheels. The tetrahedron is preferably a regular tetrahedron.
In an embodiment, the axis of rotation of a wheel is at an angle of less than 30xc2x0 relative to the corresponding edge of the tetrahedron, and preferably less than 5xc2x0.
It is also advantageous for the axes of the wheels to be fixed.
The invention also proposes a spacecraft, in particular a satellite, having such an energy storage device.
Finally, the invention provides a method of storing energy in such a device, the method comprising driving four wheels in rotation. These four wheels can, in particular, be substantially parallel to edges of the tetrahedron that are adjacent in pairs.
Preferably, the directions of rotation of the wheels are such that the angular momentum vectors all point in the same direction around a closed zigzag loop formed by the four edges.